Some Observations on the Mechanism of Inhibition

Abstract

Thiocyanic and citric acids are efficient inhibitors of dissolution of iron in dilute HNO~ solutions. It is concluded that inhibition does not generally involve "poisoning" the combination step in the hydrogen evolution reaction. Some observations on the kinetics of dissolution of iron in dilute HNO~ solutions are also presented.There is little doubt that organic compounds which inhibit dissolution of metals in acids adsorb at the metal-solution interface. There are a number of ways in which the resultant adsorbed film could modify the kinetics of the dissolution process. It has been suggested (1) that adsorbed inhibitor on iron deactivates "active centers" at which the reaction H~d~ -5 H~d~ --> H~(g) [1] between adsorbed hydrogen atoms takes place. According to this view, inhibition of iron dissolution is closely associated with occurrence of reaction [ 1 ]. Polarization studies with some organic compounds led Elze and Fischer (2) to the conclusion that inhibition of hydrogen discharge governs the process of dissolution in the presence of inhibitors. The particular inhibitors used by these authors had a negligible effect in the presence of an oxidizing agent (Fe*++). From this result, it was maintained that inhibitors function only if reaction [1] above, which according to Elze and Fischer (2) is sensitive to inhibitors, is included in the over-all dissolution process.It was pointed out (3, 4) that work of King and Hillner (5) indicates that some organic compounds function as inhibitors in strongly oxidizing solutions where reaction [ 1 ] is unimportant. However, the relevance of the work of King and Hillner (5) to the elucidation of the mechanism of inhibitor action was questioned on the ground that the observed decrease in dissolution rate may have been due to pH changes of the bulk solution (6).Makrides and Hackerman (4, 7) found that butyl thiourea inhibits dissolution of iron in FeCI~-HC1 solutions. Any bulk effect of butyl thiourea at the concentrations employed was negligible. It was of interest, therefore, to establish whether other compounds function as inhibitors in oxidizing solutions where reaction [1] may be neglected. ExperimentalThe rotating cylinder technique was used (8). Armco iron cylinders (99.92-99.96% iron content), 0.94 cm in diameter and about 8 cm long, were rotated at 250 cm/sec linear speed in 400 ml of solution. The cylinders were polished with 2/0 emery paper and were degreased with CC1,. Only the lateral surface was exposed to solution, the top and bottom parts as well as the edges being covered with paraffin. The exposed area was measured for each cylinder.Nitric acid solutions of desired concentration were made from stock by dilution. The HNO~ concentration was determined by titration. Water was doubly distilled, one distillation being from dilute, alkaline KMnO,.Stock solutions containing HSCN and citric acid were prepared by dissolving a weighed quantity of the corresponding potassium salt in an equivalent quantity of HNO~ solution. Desired inhibitor concentrations were...